Such elements are commonly used in the field of fluid regulation, since they make it possible to divide a fluid supply path into one or more distribution paths, depending on the heat of the fluid to be regulated and/or other heat sources. These elements are for example arranged within cooling circuits in which a cooling fluid circulates, in particular cooling circuits for motor vehicle heat engines or similar. Of course, other application examples can be considered, such as motor oil and gearbox circuits, as well as domestic supply water circuits.
Typically, a thermostatic element includes a metal cup with a generally tubular shape and containing a thermally expanding material such as a wax. The element also includes a piston coaxial to the cup and translatable relative to said cup under the effect of the expansion of the thermally expanding material contained in the cup, when that material is heated. In expanding, thermally expanding material partially drives the piston, such that the latter is deployed outside the cup whereas, during cooling of the thermally expanding material, the piston can be retracted inside the cup, generally under the action of a return spring associated with the thermostatic element. To guide the translational movements of the piston, the thermostatic element includes a bored metal guide, inside which the piston slides, that guide thus constituting a guide part that is firmly secured to the cup. Furthermore, to prevent the thermally expanding material from escaping outside the cup during movements of the piston and, at the same time, a liquid outside the thermostatic element, typically in which that thermostatic element is bathed, from being able to infiltrate along the piston up to the end of the piston submerged in the cup, it is known to seal the thermally expanding material relative to the outside using a flexible part whereof a tubular part is arranged coaxially around the piston. Traditionally, two main families of thermostatic elements are distinguished based on the extent of the covering of the piston by the flexible part. More precisely, when the flexible part forms a blind thimble, i.e., a sack delimiting a non-emerging cavity, the piston is received therein without establishing direct contact with the thermally expanding material: during an expansion of that material, it applies pressure forces on the sack, which then becomes pinched such that the piston is ejected therefrom, producing the translational movement. Conversely, when the flexible part is axially passed all the way through by the piston and is thus similar to a globally annular seal, centered on the axis of translation of the piston, the piston plunges directly into the thermally expanding material and is subject, without any intermediary, to the pressure applied by the material when it is heated, as illustrated by EP-A-0,940,577.
The invention specifically relates to thermostatic elements that incorporate such an annular seal and in which opposite axial parts of that annular seal are subject to pressing bearing, which is substantially antagonistic, by the guide and the cup, respectively, so as to compress the seal around the piston and thereby seal the cylindrical interface between the seal, which remains stationary relative to the guide and the cup compressing it, and the sliding piston.
During use, this sealing gasket is subject to high pressures coming from the thermally expanding material, which tend to extrude the flexible material making up the seal outside the thermostatic element, by forcing the flexible material to pass between the piston and the central bore of the guide. It is therefore known to interpose, axially between the seal and the guide, an anti-extrusion washer that is mounted coaxially around the piston.
Over the course of the usage cycles of the thermostatic element, it is observed that the flexible material making up the sealing gasket tends to wear and have permanent deformations, which gradually decreases the contact pressure between the seal and the piston, until the pressure becomes insufficient to guarantee sufficient sealing with respect to the outside of the thermostatic element. Independently of the aforementioned wear phenomenon, a critical situation may also occur when the thermostatic element is subject to a rapid and/or significant decrease in temperature, while bathing in a liquid under high pressure: under these severe usage conditions, the aforementioned liquid quite often succeeds in infiltrating along the piston, due to the accumulation of an abrupt retraction of the piston and the cup and the high pressure of the liquid. To avoid these drawbacks, it is known to attach a flexible packing, forming a sealing bellows, that is fastened to the piston and the guide, while covering the outlet thereof to the outside. That being the case, the placement of such a packing remains complex operation that is therefore expensive to implement. Furthermore, other solutions to prevent an outside liquid from being able to infiltrate the thermostatic element along the piston have been proposed in the past, but like the solution consisting of the aforementioned packing, they systematically cause an excess product cost and a non-negligible excess process cost.